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UCC27524: Slow input slope

Part Number: UCC27524

Hey team,

The datasheet of the UCC27524 (last paragraph of 8.3.3 - Input Stage) talks a bit about the implications of slow rise and fall times on the input of gate driver. It implies that slow slopes may cause repeated high-frequency switching on the output of the driver (sounds quite dangerous in power MOSFET applications). In the next sentence though, it mentions that the wide hysteresis "alleviates the concern over most other TTL input treshold devices". Also "If limiting the rise or fall times to the power device is the primary goal" (it is, i'm using a slow optocoupler as control), a gate resistor is recommended on the MOSFET. These statements leave me a bit confused. How will a gate resistor help with high-frequency switching on the output? How fast does my input slope need to be to avoid switching on the output? Shouldn't the hysteresis on the UCC27524 inputs solve this problem altogether?

Kind regards,

Lars

  • Hi Lars,

    Thanks for your interest in our driver.

    1. "How will a gate resistor help with high-frequency switching on the output?"

    The gate resistor helps dampen any oscillations at the gate due to the pulsed current from the driver's output stage. The high peak current charging the gate with little to no impedance creates an overshoot/undershoot event at the gate that can cause EMI. The copied app note further discusses the gate resistor impact at the gate.

    http://www.tij.co.jp/jp/lit/an/slla385/slla385.pdf

    2. "How fast does my input slope need to be to avoid switching on the output?" For this driver, we recommend input signals with slew rates >= 20V/us to ensure that the VIN signal "spends very little time" around the VIH and VIL thresholds of the driver as noise from ground bounce from the switching transitions or noise on the VDD supply in addition to non-ideal PCB layout can cause repeated false turn-on/off resulting in oscillations at the gates.

    3. "Shouldn't the hysteresis on the UCC27524 inputs solve this problem altogether?" The hysteresis will help to an extent as significant noise levels will be beyond the hysteresis's range.

    Please let us know if you have additional questions.

    Regards,

    -Mamadou